The measurement of opposite side surfaces of opaque parts is difficult to accomplish using conventional interferometry, particularly for purposes of both measuring the two sides individually, such as for determining flatness of each, and measuring the two sides relative to each other, such as for determining parallelism and thickness. For example, separate interferometers can be used to measure the two sides individually for determining flatness, but each is a relative measure unrelated to the other. Interferometers including combinations of mirrors have been used to measure the two sides simultaneously for determining thickness variations, but the combined measure does not distinguish differences between the sides.
The opposite sides of gage blocks and other flats have been separately measured with respect to a common mounting surface, but the results are dependent upon vagaries of the mountings. For example, a first side of the gage blocks is mounted on a flat mounting surface, and the second side of the gage blocks together with the reference surface is imaged by a multiple wavelength interferometer that measures not only variations in the second side of the gage block but also differences between the second side of the gage block and the flat mounting surface. The method assumes that the first side of the gage block is adequately represented by the reference surface on which it is mounted for purposes of measuring thickness and parallelism. However, surface irregularities, whose determination are among the actual purposes of the measurement, and foreign contaminants can distort the comparative measurements.
Another known approach arranges two long-wavelength interferometers (i.e., infrared interferometers) for simultaneously measuring the opposite side surfaces of opaque parts. For purposes of calibration, a semitransparent optical flat is located within the field of view of the two interferometers when the parts are removed and is used to define a common datum surface against which the measurements of both interferometers are referenced. Another interferometer operating in the visible range measures optical path length variations that occur subsequent to the calibration with the optical flat. Each interferometer has its own reference surface, and the calibration of the two reference surfaces with the optical flat over measurements taken at different times and under different conditions adds considerable complexity and opportunity for error.